Bluebirds face many threats living in the wild. Here are the few that endanger them the most.
With the expanding human population, the wide open spaces that bluebirds normally inhabit are being destroyed to make room for new homes, highways, farms, etc. Trees are also cut down to make room for these, and for their wood, which results in fewer and fewer places for bluebirds to nest. The bushes and shrubs that produce the wild berries and fruits they often depend upon in winter are also quickly being cleared out or eaten by other animals.
Many predators also threaten bluebirds and their nests. Some main predators include owls, climbing snakes, raccoons, hawks, squirrels, rats, and house cats. While some of these predators attack bluebirds directly, many of them also threaten their nests and young, killing fledglings and destroying eggs. It can be hard to avoid these predators, but nests, if made in nesting boxes, can be protected from climbing predators by using slick, cylindrical metal poles rather than easy-to-climb wood planks.
Two invasive species pose a huge threat to bluebirds -- European Starlings and House Sparrows. Both are very aggressive, and House Sparrows are small enough to fit into bluebird's nest holes. They will often kill or chase bluebirds and their young. While the European Starlings are a bit larger than bluebirds, and can be deterred by a smaller hole, they, like bluebirds, are cavity nesters, and can take away natural nests -- such as woodpecker holes -- that the bluebirds normally would have used. Both the European Starling and the House Sparrow have similar diets to bluebirds, and with their growing numbers often out-compete them for food. This is especially detrimental in the winter, when there's only a small supply of berries and fruits growing, and most insects have died off.